As human beings, those of us with too much time on our hands like to name things, define them, and then argue about the differences between them. How many shades of gray can dance on the head of a pin? I don't know.
But it is one thing to say there are so many shades of gray between black and white -- and something else to discuss how many shades of gray we actually see.
In the photo below you can see the smaller center rectangle is a continuum of shades of gray from a light gray on the left to dark gray on the right.
But, if I select the same inner rectangle and present it on a continuous tone background, we can see the center rectangle is actually a single shade of gray.
I think visual artists who work in two-dimensional media have always been able do their work with whatever materials they can get their hands on. An engraving or pencil drawing may use only two tones to create the illusion of many tones by cross hatching. And we perceive those tones as three dimensional objects on the two dimensional page. So the question is not so much, How many tones do you have to work with? - but, How many tones will your viewer perceive? - and, can you create the illusion you want to present? Whether we photographers have 10 shades of gray to work with (or 8 or 12), I would say there is plenty of evidence that it is enough for our purposes.